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Mud daubers are one of the few insects that lay their eggs in nests of mud. Depending on the species, the nests may look like little adobe mounds or rugged pipes. Unlike paper wasps, mud daubers are solitary; also unlike many stinging insects, they are seldom harmful to people. Only if thoroughly provoked through handling will they sting a human.
They do use their stingers on spiders, though. It is largely with paralyzed spiders, dozens packed in each chamber, that mud daubers stock their nest to feed the larvae. After filling one of the cells with food, the mud dauber lays her egg and seals the passage. Her work with that section is done, and she moves on to gathering food for the next. While it is the female mud dauber that builds the nest, in some species the male will assist in the spider-gathering, and he may also serve as a guard to prevent predators from stealing the provisions.
The designation of mud dauber includes several varieties of wasp. The blue mud dauber and the black and yellow mud dauber, both with needle-thin waists, are members of the family Sphecidae, and are related to the digger wasps. The nests that the black and yellow mud dauber builds are collections of tubes assembled into an irregularly spherical shape, the size of a small apple. The blue mud dauber often uses abandoned nests of the other mud daubers rather than building its own. It has a marked preference for black widow spiders, particularly young ones. The organ pipe mud dauber, whose name comes from the tubes of varying length that form its nest, is part of the family Crabronidae, formerly considered a subfamily of Sphecidae.
Methods of nest building vary among the varieties of mud dauber. The organ pipe mud dauber finds mud elsewhere and carries it in her mandibles to the nest site where she shapes it into tunnels. Several females may build pipes in the same cluster, which usually consists of 5 to 7 tubes. The black and yellow mud dauber uses the same technique. The blue mud dauber, however, carries droplets of water to the site and mixes it there with dirt. If it is reusing a nest, the blue mud dauber uses water to make repairs. Mud daubers can work fast, finishing a nest containing many offspring within a day. As the mud daubers build, their bodies vibrate or hum, a behavior that may help to distribute the mud. When the nest is in place, mud daubers apply a final layer of mud, which smooths and stabilizes the structure. After 9 months in the nest, new mud daubers emerge. For a few days, they are free, drinking nectar and exploring life outside the mud tunnel. Then the cycle begins again as the female mates, hunts and builds.
While mud daubers aren’t aggressive, except to spiders, the presence of their nests can cause difficulties. The narrow entrances of bat houses and some birdhouses can become impassable if a mud dauber has decided to build there. Mud daubers sometimes build nests in the small spaces of airplanes that help to determine air pressure, and these can dangerously interfere with readings.